The long-term isolation of South America during most of the Cenozoic produced a highly peculiar terrestrial vertebrate biota, with a wide array of mammal groups, among which caviomorph rodents and platyrrhine primates are Mid-Cenozoic immigrants. In the absence of indisputable pre-Oligocene South American rodents or primates, the mode, timing and biogeography of these extraordinary dispersals remained debated. Here, we describe South America's oldest known rodents, based on a new diverse caviomorph assemblage from the late Middle Eocene (approx. 41 Ma) of Peru, including five small rodents with three stem caviomorphs. Instead of being tied to the Eocene/Oligocene global cooling and drying episode (approx. 34 Ma), as previously considered, the arrival of caviomorphs and their initial radiation in South America probably occurred under much warmer and wetter conditions, around the Mid-Eocene Climatic Optimum. Our phylogenetic results reaffirm the African origin of South American rodents and support a trans-Atlantic dispersal of these mammals during Middle Eocene times. This discovery further extends the gap (approx. 15 Myr) between first appearances of rodents and primates in South America.

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We provide the first detailed description of the inner ear of a notoungulate, an extinct group of endemic South American placental mammals, based on a three-dimensional reconstruction extracted from CT imagery of a skull of Notostylops murinus. This description provides new anatomical data that should prove to be phylogenetically informative, an especially significant aspect of this research given that both the interrelationships of notoungulates and the position of Notoungulata within Placentalia are still unresolved. We also assess the locomotor agility of Notostylops based on measurements of the semicircular canals. This is the best available data on the locomotion of a notostylopid because significant postcranial remains for this group have not been described. The cochlea of Notostylops has 2.25 turns, and the stapedial ratio is 1.6. The stapedial ratio is one of the lowest recorded for a eutherian, which typically have ratios greater than 1.8. The fenestra cochleae is located posterior to the fenestra vestibuli, a condition previously only reported for some stem primates. The separation of the saccule and utricule of the vestibule is visible on the digital endocast of the bony labyrinth. The posterior arm of the LSC and the inferior arm of the PSC are confluent, but these do not form a secondary crus commune, and the phylogenetic or functional significance of this confluence is unclear at this time. Locomotor agility scores for Notostylops suggest a medium or 'average' degree of agility of motion compared to extant mammals. In terms of its locomotion, we tentatively predict that Notostylops was a generalized terrestrial mammal, with cursorial tendencies, based on its agility scores and the range of locomotor patterns inferred from postcranial analyses of other notoungulates.